Method for the correction of an error caused by variations in the sample volume in a liquid scintillation counter

ABSTRACT

The counting error due to variations in the counting efficiency as a function of the volume of the sample is corrected. The sample is placed into a sample container which is placed into a vertically positioned cylinder shaped counting chamber. Photomultiplier tubes are disposed on the opposite ends of the counting chamber. At least two of the following spectra are measured in the coincidence counting: the sum spectrum q observed by both of the photomultiplier tubes, the spectrum q(y) observed by the upper photomultiplier tube and the spectrum q(a) observed by the lower photomultiplier tube. The counting error is corrected by the correction coefficient obtained from these spectra.

BACKGROUND OF THE INVENTION

The object of the invention is a method for correcting an error due tovariations in the sample volume in a liquid scintillation counterprovided with a cylinder shaped optics. Such error is produced due tothe dependence of the counting efficiency on the sample volume. Thesample which is formed by dissolving a substance to be analyzed in ascintillation liquid and placing the dissolved substance into atransparent or translucent sample container inserted into a verticallypositioned cylinder shaped counting chamber provided withphotomultiplier tubes disposed on the opposite ends of the chamber andoperating in coincidence.

Liquid scintillation counters are commonly used for counting sampleswhich contain low energy beta or corresponding particles emittingradioactive isotopes such as tritium and carbon-14. The range of the lowenergy beta particles in the sample is generally, at the most, a fewtens of micrometers. As a consequence, the specimen to be analyzed hasto be dissolved into a scintillation liquid, in which the molecules ofthe isotope to be counted are close enough to the molecules of thescintillation substance so that the beta particles emitted by theisotope to be counted can interact with the molecules of thescintillation substance. In this interaction process, a part of theenergy of the beta particle is transformed into light, which isconverted to an electric pulse generally by means of two photomultipliertubes which operate in coincidence. The purpose of the coincidenceoperation is the elimination of thermal noise of the photomultipliertubes. The amplitude of the electric pulse is proportional to the energyof the beta particle interacted with the scintillation substance.

Because the energies of the emitted beta particles are distributed in away characteristic of the beta decay of the isotope to be counted, acontinuous spectrum corresponding to the energy distribution of theemitted beta particles is obtained by means of a multichannel analyzerincorporated in the counter. This continuous spectrum has certaincharacteristic properties e.g. total counts, number of counts in acertain "counting window" or channel region of the multichannelanalyzer, end point, maximum value and center of mass, i.e. the centroidof the obtained spectrum. It can be determined in which channel of themultichannel analyzer the end point, the maximum value and the center ofmass are located, i.e. the channel coordinates of these values can bedetermined.

A liquid scintillation counter provided with cylinder shaped optics isdefined as a liquid scintillation counter in which the transparent ortranslucent sample container containing the specimen to be analyzed andthe scintillation liquid is inserted in a vertically positionedcylindrical counting chamber with both ends open. The inner surface ofthe counting chamber itself is made of, or the surface is coated by, alight reflecting or scattering material. The purpose of the lightreflecting or scattering inner surface of the counting chamber is toguide the scintillation photons produced by the absorption of the betaparticle in the scintillation substance to the photomultiplier tubephotocathodes placed at both open ends of the cylinder shaped countingchamber.

The counting efficiency of a liquid scintillation counter means denotesthe probability of the counting system to detect the beta particlesemitted by the sample to be analyzed. It has been observed in performedexperiments that the counting efficiency of a conventional liquidscintillation counter as well as that of the liquid scintillationcounter provided with cylinder shaped optics is dependent on the totalvolume of the specimen and the scintillation liquid in the samplecontainer. As a consequence, to reach comparable results, the totalvolumes of the samples to be analyzed should always be same and in theseparate containers exactly equal.

Because it is practically impossible to keep the sample volumes alwaysequal, there is always an error in the observed count rate when thevolume of the sample deviates from an optimal value. This is due to thefact that the light collecting efficiency of the cylinder shaped opticsdepends on the optical system formed by the sample, sample container,optics and the photocathodes of the photomultiplier tubes.

SUMMARY OF THE INVENTION

The object of the present invention is to produce a novel method forcorrecting variations in the counting efficiency depending on the samplevolume, and correcting consequential counting error in a liquidscintillation counter provided with cylinder shaped optics. The methodaccording to the present invention is characterized in that the countingerror caused by the alteration of the counting efficiency as a functionof the sample volume is corrected in such a way that at least two of thefollowing spectra are measured from the sample in the coincidencecounting: the sum spectrum observed by both photomultiplier tubes, thespectrum observed by the lower photomultiplier tube and the spectrumobserved by the upper photomultiplier tube, and that the error in theresult is corrected by employing a correction coefficient obtained byutilizing the information obtained from at least two of the measuredspectra.

The inventors have observed in the performed experiments that thecounting efficiency of the liquid scintillation counter provided withcylinder shaped optics is lower for small and large sample volumes thanfor medium volumes.

Another object of the invention is a liquid scintillation counterprovided with cylinder shaped optics in order to carry out the methoddescribed above for the correction of the counting error, where theliquid scintillation counter is provided with a vertically positionedcylinder shaped counting chamber, a transparent or translucent samplecontainer which can be inserted into the counting chamber and a pair ofin coincidence operating photomultiplier tubes disposed on oppositesides (below and on the top of) of the counting chamber.

The liquid scintillation counter according to the present invention ischaracterized in that it is provided with a correction unit which hasbeen programmed a theoretically or an experimentally derived correctionfunction that corrects the obtained counting efficiency by utilizing atleast two spectra obtained from the sample in coincidence counting: thesum spectrum observed by both photomultiplier tubes, the spectrumobserved by the lower photomultiplier tube and the spectrum observed bythe upper photomultiplier tube.

Here, as in the following description of the invention, the sampledenotes the solution of the actual specimen to be analyzed and thescintillation substance, said solution being in the sample container.

Other characteristic features of the present invention will becomeapparent later in the patent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail below withreference to the attached drawings, wherein

FIG. 1 is a diagrammatic front sectional view of the counting chamber ina liquid scintillation counter provided with a cylinder shaped optics inwhich the sample container containing the sample to be analyzed isplaced;

FIG. 2 is a cross sectional view taken along the line II--II in FIG. 1;

FIG. 3 is a diagram which illustrates the counting efficiency of thesample to be analyzed as a function of the sample volume;

FIG. 4 is a diagram which illustrates the center of mass of the sumspectrum of both photomultiplier tubes and the center of masses of thespectra of the lower and the upper photomultiplier tubes as a functionof the sample volume; and

FIG. 5 is a diagrammatic front sectional view of the counting chamberconnected to the correction unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is a sample container 10 produced from atransparent material such as clear or translucent glass or plasticplaced in a counting chamber formed by a vertically positioned cylinder12. Said sample container 10 is closed by a transparent lid 11 or sealedby a transparent adhesive tape 11. Said cylinder 12 or its inner surface13 is produced from material which reflects or scatters the lightemitted from the sample 14 as well as possible in order to guide thescintillation photons emitted from said liquid scintillation sample 14as efficiently as possible to the photocathodes of the lower and upperphotomultiplier tubes 17 and 16. The liquid surface of the sample 14 isdenoted by number 15. The larger the volume of the sample 14 the highersaid liquid surface. Curve 18 in FIG. 3 illustrates the countingefficiency as a function of the volume of the liquid.

FIG. 2 illustrates the cross sectional view of a cylinder shaped opticsand the location of the sample in it.

It can be observed from FIG. 3 that the counting efficiency at firstincreases when sample volume increases but begins to decrease after acertain sample volume.

In FIG. 3

E=counting efficiency

E_(max) =maximum counting efficiency

Vol=sample volume

Vol_(max) =maximum sample volume.

FIG. 4 illustrates in coincidence counting the dependencies of thechannel coordinate q of the center of mass of the sum spectrum observedby both photomultiplier tubes, the channel coordinate q(a) of the centerof mass of the spectrum observed by the lower photomultiplier tube 16and the channel coordinate q(y) of the center of mass of the spectrumobserved by the upper photomultiplier tube 17, on the volume of thesample 14, i.e. on the height of the liquid surface 15 of said sample14.

In FIG. 4

Q=channel coordinate of the center of mass of the spectrum incoincidence counting

Q_(max) =observed maximum value of Q

q=channel coordinate of the center of mass of the sum spectrum observedby both photomultiplier tubes in coincidence counting

q(y)=channel coordinate of the center of mass of the spectrum observedby the upper photomultiplier tube in coincidence counting

q(a)=channel coordinate of the center of mass of the spectrum observedby the lower photomultiplier tube in coincidence counting

Vol=sample volume

Vol_(max) =maximum sample volume

It can be observed from FIG. 4 that the channel coordinate q of thecenter of mass of the sum spectrum at first increases slightly when thevolume of the sample 10 in sample container 14 increases but begins todecrease slightly after a certain sample volume has been reached. Thechannel coordinate q (a) of the center of mass of the spectrum observedby the lower photomultiplier tube 16 decreases, because the averagedistance of the sample 14 from the lower photomultiplier tube 16increases when the volume of the sample 14 increases. The channelcoordinate q(y) of the center of mass of the spectrum observed by theupper photomultiplier tube 17 behaves conversely: it increases when thevolume of the sample increases, because the average distance of thesample 14 from the upper photomultiplier tube 17 decreases when thevolume of the sample 14 increases.

By exploiting the dependence of the channel coordinate q of the centerof mass of the sum spectrum observed by both photmultiplier tubes andthe channel coordinate q(a) of the center of mass of the spectrumobserved by the lower photomultiplier tube 16 and the channel coordinateq(y) of the center of mass of the spectrum observed by the upperphotomultiplier tube 17 on the volume of the sample 14, the variation inthe counting efficiency caused by the variation of the sample volume canbe corrected in a correction unit (FIG. 5) by means of the followingformula:

    I.sub.c =I*1/(1-k(q)*[q(y)-q(a)])                          Formula 1:

where

I_(c) =corrected count rate of the sample

I=observed count rate of the sample

k(q) is a theoretically derived or experimental function of q.

Formula 1 corrects the observed count rate to a value corresponding tothe sample volume observed equally by both of the photomultiplier tubes.According to the experiments performed by the inventors the countingefficiency in this case is highest possible.

The operational principle of the correction method is as follows:

The correction function k(q) is stored in a computer memory of theliquid scintillation counter. It is defined theoretically or bymeasuring standard samples with different sample volumes at differentquench levels (q). From the sample to be analyzed are measured at leasta) the count rate in the counting window of the multichannel analyzer,b) the channel coordinate q of the center of mass of the sum spectrumobserved by both of the photomultiplier tubes, c) the channel coordinateq(a) of the center of mass of the spectrum observed by the lowerphotomultiplier tube and d) the channel coordinate q(y) of the center ofmass of the spectrum observed by the upper photomultiplier tube.

After this, the corrected count rate of the sample is calculated usingformula 1. It is important to notice that it is impossible to make thecorrection in question by using only the channel coordinate q of the sumspectrum of both photomultiplier tubes, although it changes slightlywhen the sample volume changes as shown in FIG. 4. The reason for thisis that the channel coordinate q of the sum spectrum also changes as afunction of the quench of the sample. Quenching means that the lightwhich can be observed outside the sample container is reduced due tochemical impurities or colourness of the solution formed by the specimento be analyzed and the scintillation substance.

The method according to the present invention is not confined to theabove illustrated example, but contains all the correction methodswithin the scope of the patent claims for correcting the variations inthe counting efficiency caused by variations in the sample volume in aliquid scintillation counter provided with the cylinder shaped optics.Namely, by means of the information illustrated in FIGS. 3 and 4 it ispossible to derive several formulas corresponding formula 1, where saidformulas do not necessarily simultaneously need q, q(y) and q(a),because for example q(a) and q(y) are behaving symmetrically as afunction of the sample volume.

What is claimed is:
 1. A method for measuring liquid scintillation,comprising:dissolving a specimen to be analyzed in a scintillationliquid; placing said dissolved specimen into a clear or translucentsample container; positioning said sample container with said dissolvedspecimen within a counting chamber having at least two photomultipliertubes on opposite ends thereof; counting liquid scintillation photons ofsaid dissolved specimen with said photomultiplier tubes to arrive at ameasured scintillation count; measuring at least two spectra bycoincidence counting of a plurality of spectra, said plurality ofspectra comprising the sum spectrum detected by both photomultipliertubes, the spectrum detected by a first one of said at least twophotomultiplier tubes, and the spectrum detected by a second one of saidat least two photomultiplier tubes; determining a correction coefficientbased on said at least two measured spectra; correcting said measuredscintillation count based on said correction coefficient.
 2. The methodof claim 1, wherein said correction coefficient is based on the centerof mass of said at least two measured spectra.
 3. The method of claim 2,wherein said measured scintillation count is multiplied by saidcorrection coefficient, said correction coefficient defined by 1/(1-k(q){q(y)-q(a)}, where k(q) is a theoretically or experimentally derivedfunction of a quench level of said dissolved specimen, q(y) is a channelcoordinate of the center of mass of the spectrum observed by a first oneof said at least two photomultiplier tubes, and q(a) is a channelcoordinate of the center of mass of the spectrum observed by a secondone of said at least two photomultiplier tubes.
 4. The method of claim3, wherein said quench level is based at least in part on at least oneof a colorness of the dissolved specimen and chemical impurities in thedissolved specimen.
 5. The method of claim 1, wherein said at least twophotomultiplier tubes comprise an upper photomultiplier tube and a lowerphotomultiplier tube.
 6. The method of claim 1, wherein said countingchamber is vertically positioned and substantially cylindrical.
 7. Themethod of claim 6, wherein said counting chamber comprises an innerreflecting surface for reflecting and scattering light within thecounting chamber.
 8. The method of claim 1, wherein said measuredscintillation count differs from an actual scintillation of saiddissolved specimen due at least in part to the volume of said dissolvedspecimen.
 9. The method of claim 1, wherein, said sample container iscovered by transparent adhesive tape or a transparent lid.
 10. A liquidscintillation counter comprising:a counting chamber; a clear ortranslucent sample container for holding a dissolved specimen to beanalyzed removably disposed within said counting chamber; at least twophotomultiplier tubes disposed on opposite ends of said counting chamberfor measuring liquid scintillation of said dissolved specimen to arriveat a measured scintillation count; a correction unit comprising meansfor correcting the measured scintillation with a correction coefficientbased on at least two spectra obtained by coincidence counting of saiddissolved specimen of a plurality of spectra, said plurality of spectracomprising the sum spectrum detected by both photomultiplier tubes, thespectrum detected by a first one of said at least two photomultipliertubes, and the spectrum detected by a second one of said at least twophotomultiplier tubes.
 11. The counter of claim 10, wherein saidcorrection coefficient is based on the center of mass of said at leasttwo measured spectra.
 12. The counter of claim 11, wherein said meansfor correcting multiplies said measured scintillation count by saidcorrection coefficient, said correction coefficient defined by 1/(1-k(q){q(y)-q(a)}, where k(q) is a theoretically or experimentally derivedfunction of a quench level of said dissolved specimen, q(y) is a channelcoordinate of the center of mass of the spectrum observed by a first oneof said at least two photomultiplier tubes, and q(a) is a channelcoordinate of the center of mass of the spectrum observed by a secondone of said at least two photomultiplier tubes.
 13. The counter of claim12, wherein said quench level is based at least in part on at least oneof a colorness of the dissolved specimen and chemical impurities in thedissolved specimen.
 14. The counter of claim 10, wherein said at leasttwo photomultiplier tubes comprise an upper photomultiplier tube and alower photomultiplier tube.
 15. The counter of claim 10, wherein saidcounting chamber is vertically positioned and substantially cylindrical.16. The counter of claim 15, wherein said counting chamber comprises aninner reflecting surface for reflecting and scattering light within thecounting chamber.
 17. The counter of claim 10, wherein said measuredscintillation count differs from an actual scintillation of saiddissolved specimen due at least in part to the volume of said dissolvedspecimen.
 18. The counter of claim 10, wherein, said sample container iscovered by transparent adhesive tape or a transparent lid.